With the recent development of portable appliances such as personal computers and cellular phones, there is a significant demand for batteries as the power sources for these appliances. Particularly, vigorous researches are being carried out in various fields on lithium secondary batteries as batteries capable of yielding a high energy density, because lithium is a substance having a low atomic weight and a high ionization energy.
On the other hand, since batteries used for these applications employ a liquid electrolyte, a problem such as the leakage of the electrolyte cannot be completely eliminated. In order to solve this problem and increase the reliability of the batteries, as well as realizing small and thin devices, attempts have been made in many fields to replace a liquid electrolyte with a solid electrolyte thereby to yield an all solid-state battery.
Particularly, in the case of the lithium secondary battery, there is a fear that the battery may heat up when the malfunction of the battery occurs because of its high energy density. Therefore, in order to ensure the battery safety, it is desired to develop an all solid-state lithium secondary battery employing a solid electrolyte comprising a nonflammable solid material. For example, lithium halide, lithium nitride, an oxyacid salt of lithium and derivatives thereof and the like are known as solid electrolytes for use in such battery.
Glassy solid electrolytes comprising a lithium ion conductive sulfide such as Li2S—SiS2, Li2S—P2S5 or Li2S—B2S3, are also known. Further, lithium ion conductive solid electrolytes, obtained by doping these glasses with lithium halide such as LiI, an oxyacid salt of lithium such as Li3PO4 or the like, are also known. Since these materials have a relatively high ion conductivity of 10−4 to 10−3 S/cm, studies have been focused on their physical properties.
For example, as described in Japanese Unexamined Patent Publication No. Hei 10-284130, an all solid-state thin film lithium secondary battery employing a solid electrolyte can be reduced in size and produced as a thin film, as well as being excellent in safety.
However, a secondary battery, particularly, an all solid-state thin film lithium secondary battery comprising a thin film electrode produced by a conventional thin film formation process, has an electrode thickness as small as 1 μm or less and therefore has a small battery capacity, so that it is not capable of providing a sufficient capacity required by appliances.
Additionally, increasing the thickness of the thin film electrode in an attempt to ensure the battery capacity does not also yield a sufficient charge/discharge characteristic required by the appliances. The reason is that, when the electrode thickness is increased, the charge/discharge characteristic decreases with an increase in the thickness.
For this reason, a secondary battery, particularly, an all solid-state thin film lithium secondary battery that is satisfactory in both the battery capacity and the charge/discharge characteristic, has not yet been produced.